Ukraine confirmed Sunday that at least 31,000 troops have so far been killed in the Russian invasion, in what is Kyiv's first official death toll since war broke out two years ago.

President Volodymyr Zelenskyy said the soldiers died defending the country from Russia's invasion, adding "I don't want to mention the number of wounded."

U.S. and Russian officials had previously said between 100,000 and 300,000 Ukrainian soldiers had been killed in the conflict, figures which Zelenskyy dismissed as "nonsense."

The Ukrainian leader meanwhile put Russian losses at 180,000 dead and 500,000 wounded.

This is the first time that one of the warring parties has released its own official casualty figures. The veracity of the toll cannot be officially verified.

The Russian casualty figures cited by Zelenskyy are significantly higher than the daily updated toll by the Ukrainian armed forces, which Sunday put the total number of Russian losses at 409,820 dead and wounded combined.

Zelenskyy declined to comment on the casualties among the Ukrainian population and said such figures are not currently known.

Exact casualties among military personnel have so far been kept strictly secret by both sides. U.S. estimates from mid-2023 put the number of Ukrainian soldiers killed at around 70,000 and the number of Russian soldiers killed at 120,000.

In mid-February, the U.S. Department of Defense estimated the number of Russian soldiers killed or wounded at 315,000.

Ukrainian prosecutors meanwhile claim they have recorded more than 120,000 instances of war crimes committed by Russian troops since the invasion started in February 2022.

"There is no crime that the Russians have not committed during this war," Prosecutor General Andriy Kostin told journalists at the Ukraine 2024 forum in Kyiv, noting that investigations had already led to 80 convictions in Ukrainian courts.

Ukraine is aiming for tribunals modeled on the Nuremberg trials held after World War II at which Nazi war criminals were tried.

Kostin cited the example of crimes committed in the southern Ukrainian city of Mariupol, which fell to Russian forces in May 2022.

On Sunday, fresh allegations emerged of Russian troops shooting seven Ukrainian prisoners of war, according to official reports from Kyiv.

The execution took place Saturday near the Russian-captured city of Bakhmut in the eastern region of Donbas, Ukrainian human rights commissioner Dmytro Lubinets said on Telegram on Sunday evening.

Lubinez referred to a video recording showing Ukrainian soldiers "with their hands up" as they surrendered.

"The Russians were supposed to take them prisoner, but instead shot them mercilessly," he said. The information from Kyiv could not initially be independently verified.

"Such an execution is a war crime," Lubinets continued. This case must be registered as a further violation of international humanitarian law by Russia, he said.

Lubinets wanted to officially appeal to the U.N. and the International Committee of the Red Cross immediately. He accused the Russian armed forces of "not caring" about the Geneva Conventions or the customs and rules of war.

Just a few days ago, Russian soldiers were accused of executing at least six wounded Ukrainian soldiers who could not be evacuated in time during the capture of Avdiivka, a town in Donetsk that had been fiercely contested. Kyiv also referred to video footage from a drone in this case.

This allegation could not be independently verified either. Russian forces have regularly been accused of executing unarmed Ukrainian prisoners of war.

Ukraine says the situation on the battlefield is being impacted by the late arrival of half of military aid from Western allies.

"Fifty percent of what has been promised does not arrive on time," Defense Minister Rustem Umerov told journalists in Kyiv, as Ukraine entered its third year of fending off a Russian invasion with Western support.

According to Umerov, Kyiv's defense campaign continues to be hampered by the lack of air superiority. Ukrainian pilots are currently undergoing training on Western F-16 fighter jets, which are due to arrive in Ukraine in the first half of the year.